1. Technical Field
The present invention relates to, for example, an electro-optical device, a semiconductor device, a display device, and an electronic apparatus having the display device.
2. Related Art
In recent years, a technology for mounting an optical sensor function on a display device, particularly, on a liquid crystal display device that uses thin-film transistors has been progressively developed, which is, for example, described in Japanese Unexamined Patent Application Publication No. 2006-118965. Three purposes to mount an optical sensor may be given: (1) measuring ambient light to adjust luminance, or the like, to thereby attempt to reduce power consumption and improve image quality; (2) measuring backlight to adjust luminance and/or chromaticity; and (3) recognizing a position of a finger or a light pen to use a touch key. The optical sensor includes a thin-film transistor, a PIN (p-intrinsic-n) diode, a PN diode, or the like. In any cases, a light receiving portion is formed of a silicon thin film, and, in order not to increase costs for manufacturing, the light receiving portion is desirably manufactured in the same manufacturing process as a silicon thin film that forms switching elements for display. When the optical sensor is formed of a thin-film transistor, a PIN diode, a PN diode, or the like, an electric current that flows through the sensor will be the sum of a photoelectric current that varies with an illuminance of light irradiated and a thermoelectric current that increases exponentially with an absolute temperature of the sensor. For this reason, in order to obtain an appropriate illuminance even at a relatively high temperature, it is necessary to effectively remove the thermoelectric current. Therefore, there is a case where a light shielding sensor that is shielded against light for thermoelectric current reference and a light receiving sensor that is not shielded against light are arranged.
At this time, for the purposes of (1) and (3), it is necessary to shield a side that is opposite to a light incident side of ambient light so that light emitted from a backlight does not enter the sensor. As a light shielding material used against backlight, for the purpose of (1), it is possible to use a metal frame, a light shielding tape, or the like, that constitutes a module when an optical sensor is located at an outer peripheral portion of a display device; however, in recent years, because of restrictions of design, or the like, it is requested to provide an optical sensor as close as possible to a display area, or inside the display area. On the other hand, for the purpose of (3), in view of its function, an optical sensor must be mounted inside the display area. Moreover, for the purpose of (2), however, it is necessary to shield an optical sensor so that ambient light is blocked and does not influence detection of illuminance of the backlight. In response to these requests, it is necessary to provide a light shielding film on the optical sensor.
When a light shielding electrode or a transparent electrode and an optical sensor are arranged so as to overlap each other, a thermoelectric current varies because of electric potential of the light shielding electrode or the transparent electrode. Thus, a thermoelectric current is not removed appropriately. In addition, when a light shielding sensor and a light receiving sensor are arranged in proximity to the display area, light emitted from the display area will partly become stray light to be irradiated to the light receiving sensor and the light shielding sensor, so that a photoelectric current by an amount of the stray light is detected as an error. Moreover, a difference in temperature occurs between the light receiving sensor and the light shielding sensor and, as a result, a thermoelectric current is not uniform. This difference in thermoelectric current is also detected as an error.